Why is it so hard to snip the apron strings?

I'd been wavering on whether or not to admit this, but as I can use the insight, here goes: As some of you know, I am (over)due to move to London to begin a new phase of living on my own. Excited? Yes, indeed, as I have never stayed there for longer than 3 months, at a time. It wasn't possible to financially afford it any earlier (during college, etc.), either.

However, I am my mother's only child, and feel markedly guilty about going away. She has always suffered from high-blood pressure. Partially due to that, she has developed a kidney condition that has been slowly worsening. She is planning to move down South to go on the state's "Transplant List" (where patients basically line up, hoping to quickly get the organ of a donor). If the match may take too long, I have offered to give her one of mine. (She refuses to accept because my recovery will take too long, but knows the offer is out there).

Her husband is of little help, and right now, she is working like the devil to afford the house she wants. Ever since my childhood, Mum has been going through difficult experiences. However, I have always been her emotional support. I can't help feeling like this is not a good time to leave her.

Simply put, I feel bad for leaving her in this state. I can't stay, as our current house will being sold by July. In London, I've got family and a new job to make my first few months an easier transition.

That's the "good, bad and the ugly" of the situation. I'm not used to being so public about something that has been private, but I can use some objective opinions.